The present invention relates to an improvement on a series compensator which is constructed by a power converter connected in series to an AC transmission line via a transformer and compensates for an electric quantity of the AC transmission line such as the voltage, current, phase or impedance.
Recently, the capacity of switching devices with intrinsic turn-off capabilities have increased and large-capacity self-commutated converters for high voltage power transmission lines to control the power thereof are being put to a practical use.
A compensator which is connected in series to an AC transmission line via a series transformer and which electrically compensates for the impedance of a power transmission line by generating a compensation voltage on the primary winding of the series transformer, thereby controlling the power flow on the transmission line, or which compensates for a variation in transmission line voltage is known as disclosed in, for example, "Static Synchronous Series Compensator: A Solid-State Approach to Series Compensator of Transmission Lines" (L. Gyugyi et al., IEEE PES 96 WM 120-6 PWRD, 1996).
FIG. 1 is a block transmission line diagram exemplifying the structure of a conventional series compensator of this type.
In FIG. 1, "G" is an AC power supply, "X1" is the transmission line inductance of an AC transmission line, "Tr1" is a series transformer, "CNV" is a power converter, "BP" is a bypass transmission line and "FL" is a harmonic filter.
The power converter CNV is structured by bridge-connecting a switching device with intrinsic turn-off capabilities like a gate turn-off thyristor (hereinafter called "GTO") and is capable of generating a voltage with an arbitrary amplitude and arbitrary frequency in accordance with the voltage and current of an AC transmission line by controlling the switching of the GTO.
The voltage generated by the power converter CNV is applied to the secondary winding of the series transformer Tr1, generating a voltage on the primary winding that is connected in series to the transmission line. The transmission line inductance X1 of the AC transmission line can be compensated by properly controlling the level and phase of the voltage generated on the primary winding of the series transformer Tr1 with respect to the voltage and current of the AC transmission line.
FIG. 2 is a vector diagram for explaining the principle of a method of compensating for the transmission line inductance.
In FIG. 2, "Vs" denotes the voltage vector of the AC transmission line, "Is" denotes the current vector of the AC transmission line, "Vc" denotes the voltage vector a power converter 4 generates on the primary winding of the series transformer Tr1, and "V1" and "V2" respectively denote the primary-side terminal voltage vector of the series transformer Tr1 on the power-supply side and the primary-side terminal voltage vector of the series transformer Tr1 on the load side.
Given that the transmission line inductance is L and the frequency of the AC power supply is .omega., the relationship between the AC supply voltage vector Vs and the primary-side terminal voltage V1 of the series transformer Tr1 is expressed by the following equation. EQU V1=Vs-j.omega.LIs (1)
The primary-side terminal voltage V1 of the series transformer Tr1 has a phase delay of .delta. and is lower by .DELTA.V with respect to the AC supply voltage Vs due to a voltage drop caused by the transmission line inductance L.
When the power converter CNV generates the compensation voltage Vc advanced by 90 degrees relative to the transmission line current on the primary winding of the series transformer Tr1, the primary-side terminal voltage vector V2 of the series transformer Tr1 on the load side changes in the direction of Vs and the phase delay and voltage drop with respect to the AC supply voltage Vs are reduced.
This is electrically equivalent to the transmission line inductance L having become smaller, and the transmission line inductance can be changed equivalently by changing the level of the compensation voltage Vc.
In general, given that the voltage at the sending end is Vs, the voltage at the receiving end is Vr and the phase difference between the voltages of the sending end and the receiving end is .theta., the maximum active power P that can be transmitted is given by the following equation. ##EQU1##
Because the maximum power that can be transmitted is inversely proportional to the transmission line inductance, the maximum transmission power can be increased by electrically compensating the transmission line inductance of a transmission line with large transmission line inductance.
In the structure in FIG. 1, as the AC transmission line and the power converter CNV are connected in series via the series transformer Tr1 in whose primary winding the same current as the transmission line current flows, the output current of the power converter CNV connected to the secondary winding of the series transformer Tr1 is constrained to the transmission line current.
When a large current flows in the transmission line due to a ground fault or the like, therefore, an excess current also flows in the power converter.
Designing the power converter so as to withstand such a large current means that a power converter having a very large capacity is used. However, the output that is needed in the normal state requires a much lower capacity such that its use is not economical.
In this respect, the bypass transmission line BP as shown in FIG. 1 is connected to the output terminal of the power converter CNV so that in case of a ground fault, the bypass transmission line BP is activated upon detection of the excess current, short-circuiting the output of the power converter. As the current constrained to the transmission line current is shifted to the bypass transmission line, the switching elements of the power converter are all turned off (gate-blocked) to prevent any excess current from flowing into the power converter.
As apparent from the above, the bypass transmission line is essential in the prior art and in case of a ground fault, the power converter should be gate-blocked to stop operation.
When the power converter is a voltage source converter as shown in FIG. 1, the current control system is generally structured to detect the output current. In a case of a series compensator, however, the output current is constrained to the transmission line current because of the above-described reason, so that current control cannot be performed.
For the series compensator, the voltage control system is designed to feedback the voltage applied to the winding of the series transformer. Since the voltage control system does not have an ability to suppress excess current that is likely to be induced by a disturbance on the transmission line side, excess current must be separately compensated.
The power converter generates a voltage with an arbitrary amplitude and arbitrary frequency by controlling the switching of the switching device with intrinsic turn-off capabilities but produces harmonics in accordance with the switching operation.
As the series compensator in FIG. 1 is connected in series to the transmission line via the series transformer, the harmonic voltage generated by the power converter is added directly to the transmission line voltage, making it essential to provide a harmonic filter like FL shown in FIG. 1.
To reduce the harmonics generated by the power converter, multiple converters should be connected.
The amount of compensation of the series compensator directly corresponds to the capacity of the power converter, so a power converter having a very large capacity to realize a large compensation amount is needed. This leads to an increase in the cost of the series compensator. Even when the transmission line inductance is large and large compensation is needed, it is preferred to restrict the compensation amount to reduce costs.
The above problems will be summarized as follows.
Because the power converter in the conventional series compensator is connected in series to the transmission line, the output current of the power converter is the transmission line current. As a result, it is necessary to provide a bypass transmission line at the output of the power converter in order to protect the power converter when excess current flows in the transmission line due to a ground fault or the like.
Since current control cannot be performed on the output current of the power converter, the excess current is likely to be induced by the disturbance on the transmission line.
As the harmonic voltage is directly applied to the transmission line, it is essential to provide a harmonic filter and multiple converters.
An increase in the compensation amount directly leads to an increase in the capacity of the power converter, so that sufficient compensation cannot be achieved.
In the meantime, protection systems for the above series compensators have the following shortcomings.
FIG. 60 exemplifies the transmission line structure of another conventional series compensator.
In FIG. 60, "1" is an AC transmission line voltage source, "2" denotes AC transmission lines, "3" is the line reactance of the AC transmission lines, "4" is a series transformer, "5" is a DC voltage source, "6" denotes a switching device with intrinsic turn-off capabilities, "7" denotes a diode, "8" is a voltage source converter which is constituted by the DC voltage source 5, the switching elements 6 and the diodes 7, "9" is a PWM control transmission line which determines the output voltage of the voltage source converter 8, "10" is a filter transmission line, "11" denotes a thyristor and "12" is a thyristor bypass transmission line including the thyristors 11.
The transmission line operation of the series compensator in FIG. 60 will now be discussed. The voltage source converter 8 generates an arbitrary AC output voltage Vo according to a switching pattern output from the PWM control transmission line 9. The AC output voltage Vo is supplied via the series transformer 4 in series to the AC transmission lines 2. FIG. 61 presents a voltage/current vector diagram when the winding ratio of the series transformer is 1:1. Given that the AC transmission line current is Is and the AC transmission line voltage is Vs, as the AC transmission line current flows through the line reactance 3, a reactance voltage VL is produced across the line reactance 3. The transmission line-voltage side terminal voltage of the series transformer 4, V1, becomes Vs+VL. As the output voltage Vo of the voltage source converter 8 can be output freely within a hatched circle in the transmission line from the center of this circle, a terminal voltage V2 on the other side of the series transformer 4 is V1+Vo=Vs+VL+Vo. The voltage component VL+Vo becomes an apparent impedance on the AC transmission lines, and controlling the voltage source converter 8 can provide the same effect as obtained by designing the line reactance 3 of the AC transmission lines variable.
The filter transmission line 10 serves to eliminates the harmonic component from the output voltage of the voltage source converter 8. The thyristor bypass transmission line 12 has each pair of thyristors 11 connected in parallel in the opposite directions, and short-circuits the windings of the series transformer 4 as the thyristors 11 are rendered conductive or enabled. When a ground fault or the like occurs in the AC transmission lines, a very large current flows through the AC transmission lines. If the thyristor bypass transmission line 12 were not used, this excess current would flow inside the voltage source converter 8 via the series transformer 4. In this respect, it is necessary to design the voltage source converter 8 so as to have a capacity large enough to endure such an excess current. This inevitably enlarges the series compensator. As the thyristor bypass transmission line 12 is used, when an excess current is produced due to a transmission line fault or the like, the excess current is made to flow through the thyristor bypass transmission line 12 by enabling the thyristors 11. During a transmission line fault, the gate of the voltage source converter 8 is blocked so that the voltage source converter 8 stops operating. It is therefore possible to design the voltage source converter 8 to function in a normal operation without considering an excess current which is generated at the time of a transmission line fault.
Because this conventional series compensator protects the voltage source converter against a transmission line fault by letting the excess current on the AC transmission lines flow through the compensation current generator thyristor bypass transmission line, the thyristor bypass transmission line must be designed as to have a capacity large enough to endure the excess current from the AC transmission lines. As a result, the thyristor bypass transmission line itself must be a large-capacity structure. In this respect, there is a demand for a series compensator which can protect the series capacitor and converter against a rising voltage and an excess transmission line current without requiring a thyristor bypass transmission line.
Further, during a transmission line fault, the thyristor bypass transmission line short-circuits the terminals of the series transformer, blocking the gate of the voltage source converter so that the voltage source converter stops operating. For the series compensator to resume the transmission line impedance compensating operation after the transmission line fault is eliminated, the thyristor bypass transmission line must be shut down before the operation of the voltage source converter is permitted. This resuming operation takes time. It is therefore desirable to provide a series compensator which can allow a compensation current generator to continuously operate even during a transmission line fault and can resume the transmission line impedance compensating operation promptly after the transmission line fault is eliminated.